1. Field of the Invention
This invention relates to a ferroelectric liquid crystal composition. More particularly it relates to a ferroelectric liquid crystal composition comprising a chiral or achiral compound having a negative dielectric anisotropy, which composition also has a negative dielectric anisotropy, and a light switching element using the above-mentioned ferroelectric liquid crystal composition.
2. Description of the Prior Art
In recent years, liquid crystal display has come to be broadly utilized as display elements, making use of specific features of thin type, light weight, small power consumption, etc. However, most of these display elements utilize TN display mode wherein liquid crystal materials having nematic phase are used so that the response thereof is still yet slow in the application fields needing a high multiplexing; hence necessity of improvement therein has been urged.
A display mode which has recently been noted in such a present status is a display mode utilizing a light switching phenomenon proposed by N. A. Clark and S. T. Lagerwall (Applied Physics Letters, vol. 36, p. 899 (1980)). The presence of ferroelectric liquid crystals has been reported by R. B. Meyer et al in the year 1975 for the first time (J. de Phys., vol. 36, p. 69 (1975), and in the aspect of liquid crystal classification, the liquid crystals belong to chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase, chiral smectic H phase, chiral smectic J phase and chiral smectic K phase (hereinafter abbreviated to SC* phase, SI* phase, SF* phase, SG* phase, SH* phase, SJ* phase and SK* phase, respectively).
When the light switching effect of ferroelectric liquid crystals is utilized for display elements, there are two superior specific features as compared with TN display mode. The first specific feature consists in that the display elements reply at a very high rate and the response time is .sup.1 /.sub.100 or less that of elements of TN display mode. The second specific feature consists in the memory effect thereof and a multiplex drive is easy as coupled with the above-mentioned high-rate response properties.
In order that display elements using ferroelectric liquid crystals are provided with memory properties, two methods are considered. One is a method proposed by N. A. Clark et al wherein the helix is untwisted by reducing the cell thickness (d) down to a thickness less than the helical pitch (p) (d&lt;p), to thereby cause the memory properties to appear (see Appl. Phys. Lett., vol. 36, p. 899 (1980)). The other is a method found by Le Piesant wherein the memory properties are caused to appear utilizing an AC-stabilizing effect (Paris Liquid Crystal Conference, p. 297, 1984).
Most of current ferroelectric liquid crystal materials have a short helical pitch; hence in order to cause the memory properties to appear by the thinning proposed by N. A. Clark et al, it is necessary to keep the cell thickness at about 1 to 3 .mu.m, but taking into account the current cell preparation technique, a problem is raised that this is difficult in the aspects of cost and yield. On the other hand, the method found by Le Piesant wherein the memory properties are caused to appear utilizing AS-stabilizing effect is effective only in the case of ferroelectric liquid crystal materials having negative dielectric anisotropy (.DELTA..epsilon.), but it is possible to cause the memory properties to appear even in a thick cell of 5 to 7 .mu.m; hence it is possible to utilize current cell preparation technique so that the method is very practical.
The AC-stabilizing effect is directed to a mode utilizing a phenomenon that in the case of a ferroelectric liquid crystal in a low frequency, the spontaneous polarization (Ps) replies to impressed electric field, while in the case of that in a high frequency, the spontaneous polarization cannot follow, and as a result, paraelectric anisotropy contributes, and if the dielectric anisotropy value is negative (.DELTA..epsilon.&lt;0), the liquid crystal molecule is compelled to be in a parallel state relative to the substrate; thus the memory properties appear even in a thick cell. The first report presenting a matrix display practically utilizing this AS-stabilizing effect is Jeary's report in 1985 (SID '85, Digest, p. 128 (1985)), but since then there has been almost no report. The main reason is that ferroelectric liquid crystal materials having a negative dielectric anisotropy are few. Further, according to Jeary's report, in order to cause the AC-stabilizing effect to appear, a voltage of about 40 V is necessary, but taking into account the driving voltage range of conventional IC, the AC-stabilizing effect is preferred to appear at far lower voltage (25 V or less). The AC-stabilizing effect appears at a lower voltage when the dielectric anisotropy has a larger negative value; hence a ferroelectric liquid crystal material having a large negative dielectric anisotropy value has been earnestly desired. Further, the response time of the ferroelectric liquid crystal material reported by Jeary et al is several msecs, that is, it is still slow in the practical aspect; hence appearance of a ferroelectric liquid crystal having a negative dielectric anisotropy value and also high-speed response properties has been desired.